The present invention is directed to a water and debris impermeable trench box panel and to the procedures for fabricating such structures. Trench boxes are structures which are dropped into trenches and earth openings in the manner of shoring and comprise plural heavy panels in paired spaced-apart adjacent register and frequently in stacked paired relation. The spaced panels are separated usually by spacer or spreader means in prevention of collapse. The object of the trench boxes is to hold out mud, water and excavated debris so that work can proceed within the boxes without endangering lives of workers in the trench or ditch. The boxes are usually assembled at the use site. By means of mechanical shovels or derricks the boxes are maneuvered into an earth opening and, as digging proceeds, the trench box is towed, nudged or urged into and along the newly prepared opening and the old opening is closed behind it over work product such as pipe or cable that has been coupled and placed in the trench by workmen. The boxes considered herein are usually made of steel and comprise at least a pair of strong panels. The panels are usually framed around the perimeter and the lower edge is usually beveled or chisel-shaped to easily penetrate and seal off the lower working face. Bearing the total weight of the panel, the chisel edge digs into the trench bottom. For the most part the outer or perimeter framing comprises a plurality of structural shapes as, for example, tubes. The perimeter frame is connected to a honeycomb interior skeleton defined by plural vertical support elements and horizontal support elements. This internal support structure, together with the perimeter frame including the chisel edge, provides a core support for inner and outer panel plates that are brought into substantially parallel register thereagainst. The plates form inner and outer skins. These skins are usually steel or aluminum sheet and the structural elements against which they are contacted are usually and correspondingly steel or aluminum and the manner of attachment is usually by welding using plug or projection welds at intervals through the skins and against the interfacing internal structural surfaces. As will be appreciated, stresses are carried through the perimeter frame and the internal skeleton reinforcing and being reinforced in turn by the attachment to the inner and outer skins. The perimeter is usually secured as by fillet welds at perimeter interphases of skins to frame and bevel or chisel edges. Thus, the trench box panels are closed as by welding but unless the most time-consuming effort is involved there are usually openings through which air, water and debris can permeate. If not present at manufacture the holes appear shortly after the trench box panels are flexed in service. These openings are exaggerated by the traumatic service that the panels receive during excavation, tamping and dragging. These stresses open the panels sufficiently so that even the soundest panels, when put into service, are unsealed and water and mud or debris enter the box and into the cavities of the panels and in time the panels of the box become useless or disabled because of the great weight of the water and accumulated mud. Not unusually the mud itself prevents draining the panel and if the panel is left in the trench, the problem is even more exaggerated.
To solve this problem the present invention fills the internal honeycomb openings or cavities between verticals and horizontal structural elements with closed cell foamed plastic material. The preferred material is lightweight foamed styrene in as close a fit as possible. The preferred technique is to use a foamed in situ procedure. This is achieved after completion of assembly and by applying foaming nozzles to one or more prepared openings through the skin and pumping the plastic and foaming agent formulation into the cavities between skins. The most economic procedure at this time, however, is the sculpting or cutting of foam blocks and the fitting of these blocks into the void spaces or cavities between skins with one skin welded in place. Then the other skin closes the sandwich and is welded to the structurals and frame parts. A combination technique is to close one side with one skin, pour foam composition into the cavities while the panel is lying flat and then cure in situ and thereafter close the panel by attachment of the inner skin. Panels of excellent quality result from any of these procedures and the expense of leak inspection is avoided and field damage to the panels is less likely to result in mud, water and debris intrusion with consequent longer life for the panels.